In processing signals used by an electronic apparatus, it is often useful to detect the peak level of a signal. This is particularly true of an electrical apparatus that is designed to read and write signals to and from a data storage medium, such as, for example, a DVD read/write device. For such apparatuses, it is important that signal peaks are accurately detected.
A peak detector is an analog circuit adapted to detect the peak levels of a signal. Conventional peak detectors detect single-end peaking signals by charging or discharging a sampling capacitor.
However, the accuracy of conventional peak detectors is limited, since the single-end peak detector architecture suffers from a low signal-to-noise ratio. Moreover, the output of conventional peak detectors is sensitive to input signal DC level variation caused by power supply variation and other noise sources. As illustrated in FIG. 1, changes in the DC level of the input signal cause the output to change. Furthermore, conventional peak detectors suffer from undesirable feed-forward noise, which corrupts the output signal when the input signal continues to change, due to capacitance coupling.
FIG. 2 is used to explain these drawbacks in greater detail. FIG. 2 shows a single-end peak detector 100 that has an amplifier 110, PMOS pair 120, sampling capacitor 130, and reset switch 140. PMOS pair 120 charges sampling capacitor 130. Reset switch 140 resets sampling capacitor 130 by providing sampling capacitor 130 with reset signal 150. The NMOS input pair formed by NMOS transistors 111 and 112 (shown in FIG. 3) are used in amplifier 110 to sense the input and output. When the output is lower than the input, amplifier 110 enables PMOS pair 120 to charge sampling capacitor 130. When the output reaches peak value, amplifier 110 shuts off PMOS pair 120 and the output is held at the peak value.
When the output has reached the peak, the input is still varying. This variation is coupled to the output through the parasitical gate-to-source capacitance of NMOS transistors 111 and 112. C1 and C2 are gate-to-source parasitical capacitors of NMOS transistors 111 and 112, respectively. The coupling effect from input to output can be represented by Equation (1):
                              Δ          ⁢                                          ⁢                      V            out                          ≈                  Δ          ⁢                                          ⁢                                    V                                                                                ⁢                                  i                  ⁢                                                                          ⁢                  n                                                      ·                                                            C                  ⁢                                                                          ⁢                  1                                +                                  C                  ⁢                                                                          ⁢                  2                                                                              C                  ⁢                                                                          ⁢                  1                                +                                  C                  ⁢                                                                          ⁢                  2                                +                Csmp                                                                        (        1        )            
Equation (1) shows that when input variation is large, or parasitical capacitor C1 and C2 are large, the output variation will be large. Moreover, this output variation is signal dependent, which results in significant nonlinearity and affects the down stream sampling circuits.
FIG. 4 illustrates the effects of parasitical capacitance. Changes in input signal 403 are coupled the output, resulting in an output signal 402 that varies from the peak value 401.